The present invention is directed to compounds that have activity as inhibitors of protein kinases. Protein kinases participate in the signaling events that control the activation, growth, and differentiation of cells in response to extracellular mediators and to changes in the environment. Inappropriately high protein kinase activity has been implicated, directly or indirectly, in many diseases. For example, high protein kinase activity can result in the failure of control mechanisms for the kinase, those related to, e.g., mutation, over-expression or inappropriate activation of the enzyme; or by over- or underproduction of cytokines or growth factors upstream or downstream of the kinase. In all of these instances, selective inhibition of the action of the kinase would be expected to have a beneficial effect.
Receptor tyrosine kinases (RTK) are generally activated by ligands that promote receptor dimerization and, in turn, autophosphorylation of tyrosine residues within the cytosolic domain. The binding of signaling proteins to these phosphorylated tyrosine residues leads to further downstream signaling. AXL family RTKs are unique in that they are activated by GAS6, a member of the vitamin K-dependent protein family that resembles blood coagulation factors rather than typical growth factors. The receptor tyrosine kinase AXL (also known as Ufo and Tyrol) belongs to a family of tyrosine receptors that includes Tyro3 (Sky) and Mer (Tyro12). Human AXL is a 2,682-bp open reading frame capable of directing the synthesis of an 894-amino acid polypeptide. Important cellular functions of GAS6/AXL include cell adhesion, migration, phagocytosis, and inhibition of apoptosis. GAS6 and AXL family receptors are highly regulated in a tissue and disease specific manner.
AXL is characterized by a unique molecular structure, in that the intracellular region has the typical structure of a receptor tyrosine kinase and the extracellular domain contains fibronectin III and Ig motifs similar to cadherin-type adhesion molecules. During development, AXL is expressed in various organs, including the brain, suggesting that this RTK is involved in mesenchymal and neural development. In the adult, AXL expression is low but returns to high expression levels in a variety of tumors. GAS6 is, so far, the single, activating ligand for AXL.
The oncogenic potential of AXL was first discovered in chronic myelogenous leukemia, but it has been demonstrated to play a role in the progression and metastasis of other cancer types. The increased expression of AXL and/or AXL ligand, Gash, has been shown in a number of human malignancies, including ovarian, melanoma, renal cell carcinoma, uterine leiomyoma, uterine endometrial cancer, thyroid carcinoma, gastric cancer, breast cancer, NSCLC, CML, AML, colorectal carcinoma, prostate cancer, various lymphomas, and esophageal cancer. The biochemical effects of increased expression of AXL are associated with increased oncogenic transformation, cell survival, proliferation, migration, angiogenesis, and cellular adhesion. Target validation studies of in vivo cancer models show that inhibition of Axl expression by RNAi blocked tumor growth in those models (see, e.g., Li, Y. et al. Oncogene 2009, 28:3442-3455).
In addition to the association with cancer and tumorigenesis, RTKs are implicated in a number of other cell and physiological functions. These include regulation of vascular smooth muscle homeostasis, platelet function, thrombus stabilization, innate immunity, and inflammation.
cMET kinase is also a receptor tyrosine kinase. HGF (hepatocyte growth factor, also known as scatter factor), the ligand for cMET, is secreted by cells of mesodermal origin whereas cMET is predominantly expressed on cells of epithelial/endothelial origin resulting in paracrine epithelial-mesenchymal cell signaling. Binding of HGF to the extracellular region of cMET activates the intracellular cMET tyrosine kinase activity.
cMET is believed to be involved in protein phosphorylation events that regulate cell proliferation, apoptosis, motility, and dissociation of cell-cell interactions, morphogenesis, angiogenesis, and epithelial-mesenchymal transition. Misregulation of cMET can lead to unregulated cell proliferation and survival. cMET is thought to be a key regulator of invasive growth, cancer tumorgenesis, and progression to metastasis. cMET gene amplification, alteration, mutation, and protein over expression or activation of cMET through autocrine or paracrine mechanisms have been detected in a wide variety of carcinomas. For example, in human gastric cancer tissue, cMET has been found to be over-expressed and amplified. In human glioblastomas and carcinomas of lung, thyroid and breast, cMET has been found to be activated as a result of increased HGF levels and autocrine signaling. In human lung cancer tissue, cMET signaling has been found to be upregulated as a mechanism of drug resistance. Activating mutations in cMET, although not as common, have been reported in sporadic and hereditary papillary renal carcinomas, head and neck squamous carcinomas as well as gastric and lung cancers. Furthermore, increased expression, the most common cMET alteration found in a wide variety of human tumors (including but not limited to renal, ovarian, hepatocellular, non-small cell lung, bone, liver metastasis of colon, oral squamous cell, esophageal, gastric, pancreatic, and prostatic cancers) correlates with poor prognosis
In summary, the AXL and cMET proteins appear to have a key role in a number of human disorders, including cancer. Thus, these proteins are an attractive and valuable target for the discovery and development of new therapeutic agents to treat cancer and other conditions. There is a need for the design of specific and selective inhibitors for the treatment of disorders mediated and/or associated with AXL and cMET.